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Description
Soil sealants are natural or artificially processed materials that
can be injected into flowing or standing water, sprayed in place
or injected into the subsurface to reduce seepage losses. They
act by filling or blocking the voids in the channel subgrade. Spraying
the sealant on the subgrade and mixing it into the soil is called
a soil modification treatment. There are a number of materials
that may be used.
A natural sealing of operating channels occurs if the water in
the channel carries considerable silt or clay-sized sediments.
Sediments penetrating the voids in the subgrade material gradually
clog the voids, thus reducing its permeability. This process is
known as sediment sealing. Sediment sealing can be performed artificially
by hydraulically or mechanically dispersing a suitable material
into water flowing at a sufficiently low velocity for deposition
to occur over the wetted perimeter of the channel over time (USBR,
1998).
The amount and method of application vary widely depending on materials.
Application methods include surface spraying, subsurface injection,
or addition to channel water. The method used depends upon the
environmental conditions at the application site and the type of
sealant used. Sealants include:
- Natural silts and clays
- Bentonite
- Resinous polymers
- Petroleum-based emulsions
- Cationic asphalt emulsions
- Sodium chloride
- Sodium carbonate or soda ash
- Sodium pyrophosphate
- Polyacrylamides
- Others including mixtures of the above materials.
Some chemical soil sealants reduce seepage by means other than
sedimentation. Chemical products applied to the channel subgrade
may have the following effects:
- React chemically to form solid or semi-solid gels.
- Deposit
precipitates in the soil voids.
- Render the subgrade impervious
to water by predominantly physical action.
An ideal channel sealant should have the following properties:
- Non-toxic to humans, animals and crops.
- Reduces seepage to
30-90L/m2/day.
- Capable of non-restrictive application, any
time of the year, under a broad range of pH and salt content,
under a
broad range
of soil composition, and in static or dynamic flow conditions.
- Resists
damage by animals, equipment, erosion and hydraulic pressure.
Durable and not subject to deterioration due to climatic
conditions, soil micro-organisms, remulsification, chemical
change or reverse
flow. Capable of resealing.
- Cost-effective.
Some of these requirements are fulfilled by commercially
available products, but a soil sealant with all of these properties
has not
been found in the literature.
The lining resulting from soil sealants is usually thin, and
therefore at risk from erosion, puncture, deterioration by weathering
and
destruction by cleaning operations (USBR, 1998). The effectiveness
of soil sealants depends on the suitability of the materials
used, the velocity of the water in the channel, and seepage mechanisms.
(McConkey et al, 1990).
Application costs are low, but long-term effectiveness and the
cost of periodically repeating the process, mean that sealants
are generally less cost-effective than other lining methods.
| Seepage
reduction |
 |
Experience with soil sealants indicates that they usually provide
a good sealing action in the first few years of service but then
rapidly deteriorate. Reductions in water losses of 65-90% have
been recorded a short time after treatment, but continuing effectiveness
is not achieved unless the treatment is periodically repeated
(Kraatz, 1977). As the cost of soil sealants is relatively low
compared
to other seepage reduction techniques, repeated applications
of soil sealants may be economically justified. Soil sealants
may
be an economic means of saving water in unlined channels during
extreme water shortages.
Indicative seepage reduction rates for some soil sealants are
presented in the table below.
Table 1 Seepage rates for waterborne soil sealants
|
| Material |
Seepage rate (L/m2/day, and % reduction) |
Reference |
|
| Natural soil |
Sealant |
Pre-treatment |
Post-treatment |
3 mths |
12 mths |
15 mths |
|
| Silty fine to medium sand |
Petroleum-based emulsion |
719 |
34 (95%) |
|
|
412 (63%) |
USBR, 1965 |
| Not specified |
Bentonite sealing |
323 |
276 (15%) |
|
|
|
USBR, 1963 |
| Sandy with some silt |
Resinous polymers |
607 |
204 (66%) |
418 (31%) |
298 (51%) |
427 (30%) |
USBR, 1965 |
| Not specified |
Compacted soil |
282 |
215 (24%) |
|
|
|
Rahimi & Bazaz, 1993 |
| Not specified |
NaCO3 |
282 |
14 (95%) |
|
|
|
Rahimi & Bazaz, 1993 |
| Not specified |
NaCO3 and asphalt emulsion |
282 |
13 (95%) |
|
|
|
Rahimi & Bazaz, 1993 |
|
| Polyacrylamides |
|
An emerging technology uses polyacrylamide-based flocculants that
bind to colloidal particles to create a relatively dense floc.
The floc settles and seals pores in the channel lining surface.
The available evidence indicates that no adverse health or environmental
effects are caused by using polyacrylamide-based flocculants.
| Biological
soil sealants |
|
Biological agents are of recent interest as a method of reducing
seepage. Micro-organisms produce extracellular sugar polymers (polysaccharides)
and form a tangled matrix of fibres that entrap material such as
silt, clay, and organic substances in a biofilm. The accumulation
of this biofilm in the soil reduces water movement through soil
pores and thus reduces permeability. The forced accumulation of
a biofilm on the wetted perimeter of a channel can therefore reduce
seepage.
There is strong evidence that growth of micro-organisms in porous
media can lead to reductions in hydraulic conductivity (Mitchell
and Nevo 1964, Frankenberger et al., 1979 and Ragusa et al., 1994).
Laboratory experiments using columns packed with irrigation channel
soil confirmed linear relationships between polysaccharide production
and reductions in hydraulic conductivity. These same experiments
showed that algae were involved in clogging of soil pores. Field
experiments showed that factors that reduced growth of algae (e.g.
total suspended solids) led to a reduction in polysaccharide content
in irrigation channel sediment.
Irrigation channels provide a nutrient-rich habitat for benthic
micro-organisms, and clogging of channel beds with polysaccharide-producing
benthic algae could prove to be a low-cost technique to control
seepage from channels. However, some channels may be too low in
nutrients for inoculation with benthic micro-organisms to be successful.
The Institute of Sustainable Irrigated Agriculture in Tatura, Victoria
assessed algal inoculation of channels to reduce seepage. It was
estimated that reduction in seepage rates of around 20% could be
expected for algal inoculation (McLeod, 1993).
| Related
pages |
|
Earthen lining techniques
Compacted earthen liners
Clay
lining example: Channel 12
Clay
lining example: Waranga Western Channel
Other
Australian examples
Channel bank remodelling
Loose earthen linings
Bentonite treatments
Modified soil earthen linings |
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